Method of manufacturing lithoprinting plate

ABSTRACT

PROCESS FOR PRODUCING A LITHOPRINTING PLATE IN WHICH A LATENT ELECTROSTATIC IMAGE IS PRODUCED ON A PHOTOCONDUCTIVE PLATE ON WHICH THE PHOTOCONDUCTIVE SURFACE LAYER HAS A CONTACT ANGLE WITH WATER OF FROM 135* TO 180* AT 20*C. AND 65% RELATIVE HUMIDITY, DEVELOPING THE IMAGE WITH A LIQUID DEVELOPER, APPLYING AN ELECTRIC CHARGE AND ETCHING.

United States Patent U.s. Cl. 961 4 Claims ABSTRACT OF THE DISCLOSURE Process for producing a lithoprinting plate in which a latent electrostatic image is produced on a photoconductive plate on whichthe photoconductive surface layer has a contact angle with water of from 135 to 180 at 20 C. and 65 relative humidity, developing the image with a liquid developer, applying an electric charge and etching.

BACKGROUND OF THE INVENTION (A) Field of the Invention This invention relates to improved methods for the production of lithoprinting plates from photoconductive plates.

(B) Description of the Prior Art Lithoprinting plates are normally prepared from electrophotographic copying materials, referred to herein as photoconductive plates, which comprise an electrically conductive support having a photoconductive surface layer. The surface layer typically contains a photoconductive material dispersed in an electrically insulating resinous or' polymeric. binder. The photoconductive plate is converted to a lithoprinting plate by a process comprising the steps of:

( 1) Charging the photoconductive surface by exposure to a high voltage corona discharge,

(2) Selectively leaking the charge by exposure to light reflected from or passed through the image to be reproduced to develop a latent electrostatic image,

(3) Developing the image with a liquid developer comprising a toner dispersed in a hydrocarbon liquid having a high specific resistance of about 10 ohms per .cm. and a dielectric constant of 3 or less,

,(4) Desensitizing the non-image area of the offset master so produced through etching.

A difiiculty with this procedure is that the liquid developer fails to adhere closely to thelatent image area with the result that the etching liquid infiltrates and partially desensitizes the image area. As a result, the printing ink fails to spread well on the image area, and the printed images produced from the lithographic plates are of low concentration. Attempts to alleviate this problem, for example by the use of weaker etching liquids have re sulted in imperfect desensitization of the non-image areas of the lithopr'inting plate. Printed "images produced from such imperfect plates have stained backgrounds.

SUMMARY OF THE INVENTION This invention markedly alleviates the difficulties .of conventional methods, and provides a procedure for manufacturing lithoprinting plates capable of producing printed images of high concentration which are substantially free from background stain.

The principal features "of the invention are the selection of photoconductive' plates iii-which the photoconductive surface layer has a contact angle with water which is 3,834,909 Patented Sept. 10, 1974 ice from to at 20 C. and 65% relative humidity, and the application of an electric charge to the developed image prior to etching.

Broadly speaking, the invention comprises the steps The latent electrostatic image is produced in the conventional manner.

-As a result of the process desribed above, it has been found that offset masters are produced in which the nonimage areas are easy to wet with etching liquid. The image area, on the other hand, does not accept the second electric charge so that it maintains a large contact angle with water, and is substantially free of attack by the aqueous etching liquid. If the contact angle of the photoconductive surface is appreciably less than 135, even the use of the second electrical charge in accordance with this invention will not protect the image area from attack by the etching fluid.

A particular advantage of the invention is that the second charge can be imparted to the plate in the same manner using the same equipment as the first charge. It is, moreover, generally of the same order of magnitude.

In preferred embodiments of this invention, conventional polymeric binders and photoconductive substances are employed except that they are modified by mixing or coating with surface modifiers to insure that the surface will have the right contact angle. The surface modifiers utilized in the invention are liquids at ambient temperature (about 20 C. to 45 C.), of low volatility and may be compounded with or coated. on the photoconductive layer. Suitable treating agents include, for example:

(I) Silicone oils having the following structural formula:

wherein R represents hydrogen, alkyl radical contalnmg 1-6 carbon atoms, allyl radical containing 16 carbon atoms; and n31, at least on R being other than hydrogen. (II) Esters including:

(a) Glycerides having the following structural formula:

CHQOCORI CHOCOR;

cHZOGOR,

wherein R R and R representj fatty acid residual radicals which may be the same or different, saturated or unsaturated and contain up to about 18 carbon atoms. This includes, falr) instance, triglyceride of -oleic acid (sesame o1 f I (b) Monohydric alcohol esters of such fatty acids as suberic' acid, azelaic acid, containing up to about ten. carbon atoms in the acid moiety; and six carbon atoms in the: alkoxy moiety, i (c) Monohydric alcohol esters of monocarboxylic aliphatic and aromatic acids and fludrinated 'alcohols, i f 1 (d) Monohydric alcohol esters of monocarboxylic aliphatic and aromatic acids and phosphoric acids, (III) Polybutene,

3 (IV) Dialkylated naphthalenes, such as 1,4-dialkylated naphthalenes wherein the alkyl radical contains 1-5 carbon atoms,

(V) Alkylated diphenyls wherein the alkyl radical contains 1-6 carbon atoms, for instance, Arocizer, a manufacture of Shin Nihon Seitetsu Kagaku K.K.,

(VI) Mineral oils, and

(VII) Liquid paraffins The foregoing materials for improvement may be applied with the materials constituting the photoconductive layer. In the case of applying it by blending with the materials constituting the photoconductive layer (wherein an inorganic material is employed as the photoconductor) or recording material, it is appropriate to employ said material for improvement to the extent of 0.550 parts by weight per 100 parts by weight of the resinous binder employed. The preferred range is from 1 to 30 parts by weight per 100 parts. In these cases, the quantity of inorganic photoconductor in the photoconductive layer is desirably in the range of 100-800 parts by weight per 100 parts by weight of the resinous binder.

The electrically insulating polymers utilized in this invention may be selected from any of a large number of conventional polymers and copolymers including, for example, acrylic resins, polyvinyl acetates, alkyd resins, cellulose resins, phenol resins, polyesters, and the like.

The photoconductive materials which may be utilized include, for example, zinc oxide, cadmium sulfide, zinc sulfide, amorphous selenium, titanium dioxide and the like. Sensitizers such as Rose Bengal may be added in the usual amounts to improve the spectral sensitivity.

The photoconductive plate may be treated on the side opposite the photoconductive surface with any of the usual materials used to improve electrical conductivity such as polyvinyl benzyl ammonium chloride. There may also be a hydrophilic layer present under the photoconductive layer such as starch, casein, polyvinyl alcohol and the like. Both of these possibilities are in accordance with conventional practice.

It is not necessary to employ special etching fluids in the practice of this invention. Any of the commercially available fluids can be utilized.

Typical liquid developers which can be used to develop the latent electrostatic images of this invention include, for example, those described in U.S. Pat. No. 3,554,946. Any of a wide variety of others may also be used.

The following non-limiting examples are given by way of illustration only:

EXAMPLE 1 Water-soluble epoxy resin (l%-aqueous solution)- Styrene-butadiene resin latex (solid: 50%) 50% aqueous dispersion of clay 66 Water 100 Next, by coating a conductive liquid having the following composition on the back of the same paper and drying at 130 C., a conductive layer weighing 10 g./m. was formed thereon.

Parts by weight 60% aqueous solution of polyvinyl alcohol 100 ECR-34 (30% aqueous solution) =(polyvinyl benzyl trimethyl ammonium chloride, a manufacture of Dow Chemical 00., Ltd.) 10 50% aqueous dispersion of clay 26 Water 100 Subsequently, by subjecting the thus processed paper to super calendering, a support was prepared. Then, by

coating a photoconductive dispersion having the following composition on the precoated surface of this support and drying at 130 C., a photoconductive layer weighing 30 g./m. was formed thereon.

Parts by weight Zinc oxide 100 Acryl resin (50% toluene solution of methyl and butyl methacrylate, mole ratio 50:50 36 Silicone varnish (70% toluene solution) 3 Silicone oil '(FK-99 manufactured by Shinetsu Kagaku Co., Ltd.) 0.8 5% methanol solution of Rose Bengal 1 Toluene 120 etching. The etching liquid spread well on the non-image area to desensitize it, while the image area was free of adhesion of etching liquid and was not desensitized.

When this offset master was employed for printing by means of a commercial offset printing machine, there were obtained clear, concentrated, distinct prints free of background stain (concentration of the image area: 1.20).

On the other hand, in the case of an offset master prepared by omitting the process of applying corona discharge prior to etching, there was background stain due to adhesion of ink.

A comparative copying material not containing silicone oil in its photoconductive layer was also prepared in the same way as above. The contact angle with water of this copying material was 120. When an otfset master prepared by the use of this copying material was employed for printing, concentration of the image area was as low as 0.55.

Measurement of the contact angle was conducted by the use of a goniometer-type contact angle measuring instrument in the atmosphere of 20 C. in temperature and 65 in R.H.

EXAMPLES 2-4 Three photoconductive plates having a contact angle with water of 145, 140 and 150, respectively, were prepared by appyling 5 parts by weight of sesame oil, 6 parts by weight of linseed oil and 4 parts by weight of mineral oil, respectively, in lieu of silicone oil employed in Example 1. When these copying materials were utilized as offset masters in the same manner as in Example 1, there were obtained prints high in concentration in the image area and free of background stain.

What is claimed is:

1. A method for manufacturing a lithoprinting plate comprising the steps of:

(a) forming a latent electrostatic image on a photoconductive plate, said plate being characterized by the presence of a photoconductive surface layer having a contact angle with water of from to at 20 C. and 65% relative humidity,

(b) developing the image with a liquid developer,

'(c) applying an electric charge to the nonimage areas of the developed surface,

'(d) etching the charged surface.

2. A method according to Claim 1 wherein the photoconductive surface layer comprises an electrically insulating polymeric binder, a photoconductive substance and, for each 100 parts by weight of said binder, from 0.5 to

wherein R represents hydrogen, alkyl radical containing 1-6 carbon atoms, allyl radical containing 1-6 carbon atoms; and n 21, at least one R being other than hydrogen, (II) esters including:

(a) glycerides having the following structural formula:

CHzOCOR;

HOCORI CH OCOR;

wherein R R and R represent fatty acid residual radicals which are the same or different, saturated or unsaturated and contain up to about 18 carbon atoms.

Kb) monohydric alcohol esters of dicarboxylic acids containing up to about ten carbon atoms in the acid moiety and six carbon atoms in the alkoxy moiety,

(c) monohydric alcohol esters of monocarboxylic aliphatic and aromatic acids and fluorinated alcohols,

(d) monohydric alcohol esters of monocarboxylic aliphatic and aromatic acids and phosphoric acids,

(III) polybutene (VI) Dialkylated naphthalenes wherein the alkyl radical conditions 1-5 carbon atoms '(V) alkylated diphenyls wherein the alkyl radical contains 1-6 carbon atoms,

(VI) mineral oils, and

(VII) liquid paraflins.

4. A method according to Claim 2 wherein the photoconductive substance is zinc oxide.

References Cited UNITED STATES PATENTS 3,245,381 4/1966 Brenneisen 961 LY 3,472,676 10/ 1969 Cassiers et al 11737 LE 3,486,922 12/1969 Cassiers et al. 117-37 LE RONALD H. SMITH, Primary Examiner J. P. BRAMMER, Assistant Examiner U.S. Cl. X.R.

96-1 R; 101-463, 465, 467; 117-37 LE 

